The present invention relates to a wide-band single mode optical fiber coupler having reduced wavelength dependence of its splitting ratio and a method of making such an optical fiber coupler.
A conventional single mode optical fiber coupler of low wavelength dependence of its splitting ratio is produced by fusing together stretched and unstretched optical fibers of different propagation constants, as disclosed in PCT Publication W087/00934.
FIG. 1 shows the wavelength dependence of the splitting ratio of an optical fiber coupler manufactured using such single mode optical fibers of different propagation constants. FIG. 2 shows, for comparison, the wavelength dependence of the splitting ratio of an optical fiber coupler fabricated using optical fibers of the same propagation constant.
As will be seen from FIGS. 1 and 2, the splitting ratio of the latter optical fiber coupler varies sinusoidally between 0 and 100% with respect to the wavelength and its maximum value is 100%, whereas the splitting ratio of the former is maximum at a wavelength .lambda..sub.1 and its value is smaller than 100%. Consequently, either of the two optical fiber couplers can be produced so that the splitting ratio at the wavelength .lambda..sub.1 may be 50%, for instance, but the wavelength dependence of the splitting ratio in the vicinity of the wavelength .lambda..sub.1 is more reduced and more flattened in the case of the former optical fiber coupler whose splitting ratio becomes maximum at the wavelength .lambda..sub.1, than the wavelength dependence of the latter.
Conventionally, an optical fiber coupler having reduced wavelength dependence of the splitting ratio is produced by designing it so that the splitting ratio reaches a desired peak value at its center wavelength, through utilization of the effect that the maximum value of the splitting ratio becomes smaller than 100% in the optical fiber coupler fabricated using optical fibers of different propagation constants.
In the prior art optical fiber coupler, however, the wavelength dependence of the splitting ratio is not sufficiently reduced, because its reduction relies solely on the idea of making the maximum value of the splitting ratio become a desired value smaller than 100% at the center wavelength.